The point is that these petrol-powered bikes well exceed the performance of a pedal bicycle under a fit and healthy adult male. As such, the consequences of their misuse become much greater. I have no problem with them, per se, but they need ot be properly classified and treated under the law ie, as motorcycles because that is what they have become.
The electric assist bike that Joeblake describes is perfect as a power-assist bike. I have no problem with it, and it's a good fit for the scenario you set out lycra_hero.
On the other hand, the 2-stroke-powered POS that I see blast past uphill at 40km/hr along Allambie Road at 7:50 each morning is exactly the sort of thing that needs to be banned or forced to comply with registration and licensing requirements. The guy looks scruffy and dirty and has a drinkers complexion, so I'm guessing he's resorted to this thing because he's lost his licence for DUI.